<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
<style>

body { margin: 0; }
canvas { width: 100vw; height: 100vh; display: block; }


</style>
</head>
<body>

<canvas id="canvas"></canvas>

<!-- vertex shader -->
<script id="vs" type="f">
attribute vec2 position;
attribute vec2 texcoord;

uniform mat3 u_matrix;

varying vec2 v_texcoord;

void main() {
   gl_Position = vec4(u_matrix * vec3(position, 1), 1);
   v_texcoord = texcoord;
}
</script>
<!-- fragment shader -->
<script id="fs" type="f">
precision mediump float;

uniform float u_time;
uniform float u_distortionAmount;
uniform float u_distortionRange;
uniform vec2 u_mouse;

// our textures
uniform sampler2D u_originalImage;
uniform sampler2D u_mapImage;

// the texcoords passed in from the vertex shader.
varying vec2 v_texcoord;

void main() {
   vec4 depthDistortion = texture2D(u_mapImage, v_texcoord);
   float distortionMult = depthDistortion.g;     // just green channel
   float parallaxMult = 0.5 - depthDistortion.r; // just red channel
   
   vec2 distortion = vec2(
      sin(u_time + v_texcoord.y * u_distortionRange), 0) * u_distortionAmount  * distortionMult;
   vec2 parallax = u_mouse * parallaxMult;
      
   vec4 color0 = texture2D(u_originalImage, v_texcoord + distortion + parallax);
   gl_FragColor = color0;
}
</script>
<script src="https://twgljs.org/dist/3.x/twgl-full.min.js"></script>
<script src="https://webglfundamentals.org/webgl/resources/m3.js"></script>


</body>
<script>

"use strict";

function main() {
  // Get A WebGL context
  /** @type {HTMLCanvasElement} */
  const canvas = document.getElementById("canvas");
  const gl = canvas.getContext("webgl");
  if (!gl) {
    return;
  }

  let originalImage = { width: 1, height: 1 }; // replaced after loading
  const originalTexture = twgl.createTexture(gl, {
    src: "https://i.imgur.com/xKYRSwu.jpg", 
    crossOrigin: '',
  }, (err, texture, source) => {
    originalImage = source;
  });
  
  const mapTexture = twgl.createTexture(gl, {
    src: "https://i.imgur.com/W9QazjL.jpg", crossOrigin: '',
  });

  // compile shaders, link program, lookup location
  const programInfo = twgl.createProgramInfo(gl, ["vs", "fs"]);

  // calls gl.createBuffer, gl.bindBuffer, gl.bufferData for a quad
  const bufferInfo = twgl.primitives.createXYQuadBufferInfo(gl);

  const mouse = [0, 0];
  document.addEventListener('mousemove', (event) => {
    mouse[0] = (event.clientX / gl.canvas.clientWidth  * 2 - 1) * 0.05;
    mouse[1] = (event.clientY / gl.canvas.clientHeight * 2 - 1) * 0.05;
  });
  
  requestAnimationFrame(render);
  
  function render(time) {
    time *= 0.001;  // seconds
    
    twgl.resizeCanvasToDisplaySize(gl.canvas);

    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

    gl.clearColor(0, 0, 0, 0);
    gl.clear(gl.COLOR_BUFFER_BIT);

    gl.useProgram(programInfo.program);

    // calls gl.bindBuffer, gl.enableVertexAttribArray, gl.vertexAttribPointer
    twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);

    const canvasAspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
    const imageAspect = originalImage.width / originalImage.height;
    const mat = m3.scaling(imageAspect / canvasAspect, -1);
    
    // calls gl.activeTexture, gl.bindTexture, gl.uniformXXX
    twgl.setUniforms(programInfo, {
      u_matrix: mat,
      u_originalImage: originalTexture,
      u_mapImage: mapTexture,
      u_distortionAmount: 0.003,  // .3%
      u_distortionRange: 100,
      u_time: time * 10,
      u_mouse: mouse,
    });

    // calls gl.drawArrays or gl.drawElements
    twgl.drawBufferInfo(gl, bufferInfo);
    
    requestAnimationFrame(render);
  }
}

main();


</script>
